Another M125 Build

KT88's



All important GFI



As a complete novice I really appreciated all the great help from Bob Latino and members of this forum. The final result is highly satisfying; listening nirvana.

Things did start out a bit rocky. Went through 10 fuses and two 5AR4 tubes within the first few weeks. If a rectifier even burps it takes out the fuse.  I kept wondering, why is this happening? Turns out there are several rectifiers one can select, some better then others.  Solid state rectifiers like the Weber WZ68 are inexpensive and sturdy. Tube rectifiers are a different animal. Certain older NOS ones such as GZ37, GZ33 have a good reputation for reliability but are expensive. I recently applied the "diode" mod to improve stability when using a current production tube rectifier like the GZ34/5AR4, except I installed heavier duty UF5408 diodes versus IN4007. There is a sticky on this subject on the front page for those interested in tube rectification.

I keep the amps cool with 4 Noctua NFP12 fans connected to a fan controller. At one point had them sitting on the top shelf of my component console but folks kept wanting to touch them!

Anyway, totally enjoying these fabulous amps!

Looks nice How much bias did you give the tubes, and does it get so hot
inside that you need the fan ? I actually have the same fan in two of my
other amps but I thought that I wouldn't need one for these amps
since almost everything is on the outside.

Dahlberg wrote:Looks nice How much bias did you give the tubes, and does it get so hot
inside that you need the fan ? I actually have the same fan in two of my
other amps but I thought that I wouldn't need one  for these amps
since almost everything is on the outside.

Bias = 1 volt. The fans and fan controller are from a few years when I was into computer water cooling. (stupid hobby/waste of money). With the fans running, even following extended sessions as higher volumes, I am still able to physically touch the power transformers. Otherwise they get too hot to touch. Not sure how hot is too hot?
   
Overall setup below. Marantz PM11S1 integrated serving as preamp, modded Rega TT, Cambridge CD player. Also Polk LsiM speakers and a SVS 12" sealed sub.

Ok, and 1volt equals 100mA x 4 kt88's right ? Not sure about this measuring of two tubes at ones

Edit: Beautifull setup by the way

Dahlberg wrote:Ok, and 1volt equals 100mA x 4 kt88's right ? Not sure about this measuring of two tubes at ones

Edit: Beautifull setup by the way

Thanks for compliment! I do enjoy it. Sound quality has really moved to next level because of M125's. This is comparing to ~$4k SS amp, what the Marantz originally is retailing at. (still using because it has excellent MC phono)

100 ma figure I have no idea abut this, I am novice when it come to tube amps.

A question about the fans. Are they located outside of the amps under the mid
shelf blowing downwards or did you manage to cut the round hole required in
the stainless steel bottom ? If so, how was that accomplished ?

Dahlberg wrote:A question about the fans. Are they located outside of the amps under the mid
shelf blowing downwards or did you manage to cut the round hole required in
the stainless steel bottom ? If so, how was that accomplished ?

The first one, except blowing air from behind the amps. I position them so they blow air across and over top of power transformer. Design of my console stand made this easy to accomplish. I fashion simple brackets to attach the 120 mm fans, these in turn attach to frame of console with screws. The wires I run into a small 4 position fan controller originally meant for 3.5" computer bay. This I also attach to frame of console. Speed control knobs point upwards and are readily accessible from the rear edge of console. I set fan speed very low so fans run quiet. But still is working really well to control temperatures. Here is picture from different angle:



I see your post in main forum. I think GZ37's would look very nice in these amps! Maybe you can post some pics at some point?

Edit: If amps are not within an enclosed space then fans are not necessary.

LeGrace wrote:

Dahlberg wrote:A question about the fans. Are they located outside of the amps under the mid
shelf blowing downwards or did you manage to cut the round hole required in
the stainless steel bottom ? If so, how was that accomplished ?

The first one, except blowing air from behind the amps. I position them so they blow air across and over top of power transformer. Design of my console stand made this easy to accomplish. I fashion simple brackets to attach the 120 mm fans, these in turn attach to frame of console with screws. The wires I run into a small 4 position fan controller originally meant for 3.5" computer bay. This I also attach to frame of console. Speed control knobs point upwards and are readily accessible from the rear edge of console. I set fan speed very low so fans run quiet. But still is working really well to control temperatures. Here is picture from different angle:



I see your post in main forum. I think GZ37's would look very nice in these amps! Maybe you can post some pics at some point?

Edit: If amps are not within an enclosed space then fans are not necessary.

Ok, thanks for the description. My amps will be placed in a fairly open space so they will probably be ok then.
I got the amps today and I will post pictures as they get put together. I will post a new thread then.

Look forward to seeing pics! Here is some advice from my build experience. When I assemble one of the amps I initially have a problem with hearing AC hum through speakers, an annoying sound! But then I go back and isolate twisted wire pairs away from other wires and then hum problem go away completely. The amps when properly assembled should be completely silent.